A catalyst is provided to an exhaust pipe of an internal combustion engine to purify exhaust gas. In a conventional operation, secondary air is supplied to the upstream of a catalyst using an air pump to enhance purification efficiency of the catalyst.
When secondary air is supplied, a fuel injection amount is controlled such that the air fuel ratio (combustion air fuel ratio) of mixture gas, which is supplied to the engine, becomes high to the rich side. The air fuel ratio (A/F ratio) is detected using an air fuel ratio sensor (A/F sensor) arranged in the vicinity of an inlet of the catalyst, so that the A/F ratio is controlled in accordance with the detection signal of the A/F sensor. However, in this situation, the injection amount of fuel may be excessively increased. That is, when secondary air is supplied, the amount of fuel injection is increased in accordance with a flow amount of secondary air. Subsequently, when supply of the secondary air is stopped, the amount of fuel injection quickly changes. For example, when the flow amount of secondary air temporarily increases or temporarily decreases corresponding to change of an operating condition of the engine, the amount of fuel injection may be excessively increased. As a result, drivability may be deteriorated, and emission of exhaust gas may increase.
Therefore, when secondary air is supplied, feedback control of the A/F ratio may be prohibited to evade such deterioration in drivability and increase in emission. According to JP-B2-2910034, when secondary air is supplied, and an A/F sensor outputs a rich signal, feedback control of the A/F ratio is operated. Besides, when secondary air is supplied, and the A/F sensor outputs a lean signal, feedback control of the A/F ratio is prohibited.
However, the injection amount of fuel is preferably compensated, that is, the A/F ratio is preferably controlled in accordance with increase and decrease in flow amount of secondary air for maintaining exhaust emission in a favorable condition, even when secondary air is supplied. Therefore, the amount of fuel injection needs to be compensated when secondary air is supplied.